Medical treatments for disorders of the nervous system, such as seizure disorders (e.g., epilepsy), have improved in recent decades. One available treatment involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including a reduction of seizure occurrence and the improvement of other medical conditions. An example of such a treatment regimen involves the application of electrical stimulation to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. No. 4,702,254, which is incorporated herein by reference.
Electrical stimulation of a target tissue of a patient's body may be provided by implanting an electrical device (known as an implantable medical device, or “IMD”) underneath the skin of a patient and electrically stimulating the target tissue. In some cases, electrical stimulation of target tissue (including, but not limited to neural tissue such as the vagus nerve) may be delivered in accordance with a programmed (or predetermined or “planned”) schedule. In such cases, the electrical stimulation is referred to as “open-loop,” “passive,” “programmed,” or “non-feedback” stimulation. In other cases, electrical stimulation may be delivered in response to detecting some type of event. In one embodiment, the event may be patient-initiated, i.e., the patient may manually initiate stimulation by performing an action that is detected as an event (“manually-initiated” or “manually-requested” stimulation). In another embodiment, a change in one or more body parameters (for example, cardiac rhythm, muscle activity, or body movements) may be detected as an event that triggers electrical stimulation. Typically, the body parameter(s) is selected such that the change in the parameter is indicative of a disease state such as an epileptic seizure. Stimulation initiated based on a detected change in a body parameter is referred to as “automatic” or “automatically-initiated” stimulation. Event based stimulation, which includes manually initiated and/or automatic stimulation, is known as “closed-loop,” “active” or “feedback” stimulation. In some devices, both open-loop and closed-loop stimulation may be simultaneously employed, with an open-loop program operating to provide a basic level of therapy and closed-loop stimulation provided in response to episodic events.
Whether delivered as closed-loop or open-loop, the stimulation is typically applied as a sequence of pulses (collectively referred to as a “burst”) extending for a defined duration (known as the “on-time” or “burst duration”). In open-loop stimulation, the pulse bursts are separated by a programmed time period (the “off-time”), and in closed-loop stimulation the bursts are delivered in response to the detected event and may include a refractory period after the closed-loop burst to allow the nerve to recover. During the on-time of a pulse burst, electrical pulses of a defined electrical current (e.g., 0.5-3.5 milliamps) and pulse width (e.g., 0.25-1.0 milliseconds) are delivered at a defined frequency (e.g., 20-30 Hz) for the burst duration (e.g., 7-60 seconds). For open-loop stimulation, the on-time and off-time parameters together define a duty cycle, which is the ratio of the on-time to the combination of the on-time and off-time, and which describes the percentage of time that the electrical signal is applied to the nerve.
Most IMDs are powered by onboard batteries; consequently, the amount of power available is finite. Just before the battery of an IMD is exhausted, the IMD must be surgically removed from a patient's body so that a new device (or battery) may be installed. For this reason, the ability to accurately predict a battery's remaining life is crucial to ensuring that therapy to the patient is not interrupted, and to avoid endangering the patient's health. Overestimating an IMD's battery life can result in the undesirable interruption of therapy caused by not replacing the IMD and/or battery prior to exhaustion of its electrical charge. On the other hand, underestimating an IMD's battery life can result in surgery that is not then necessary, and a waste of the useful life of the IMD.
Predicting battery life generally is relatively uncomplicated when an IMD only applies electrical pulses in accordance with a planned schedule (“open-loop” stimulation). When closed-loop stimulation is used (either alone or in combination with open-loop stimulation), predicting battery life becomes difficult, because closed-loop stimulation is patient-specific and does not occur according to any predetermined schedule.
Thus, methods and systems for accurately predicting battery life in IMDs that apply closed-loop stimulation are desired.